1. Technical Field
The present invention relates to a spray drying process and an apparatus therefor. More particularly, the present invention relates to a spray drying process and an apparatus therefor, both using a spray drying chamber formed by a porous membrane.
2. Background Art
Spray drying processes and apparatuses therefor basically have three essential functions, namely, (1) spraying of a feed liquid to form microdrops, (2) drying of the microdrops to produce a powder and (3) separation and recovery of the powder product. These spray drying apparatuses are ordinarily equipped with an atomizer, a drying chamber and a fine powder collector to perform each of said functions.
As examples of the spray drying apparatuses, there have conventionally been popular those shown in FIG. 9 and FIG. 10. (Reference is made to Japanese Patent Publication No. 32601/1983 and Japanese Utility Model Application Kokai (Laid-Open) No. 26950/1983.)
In the spray drying apparatus of FIG. 9, in a drying chamber 1, a feed liquid is sprayed from a nozzle 5 and momentarily heated by hot gas (heated air) blown from a hot gas inlet 2, whereby the liquid component of the feed liquid is evaporated and the solid component is converted to a fine (dried) powder. The majority of the fine powder is taken out as a product through a rotary valve 6, but part of the fine powder is carried by an exhaust gas into a cyclone collector (hereinafter referred to as "a cyclone") through a discharge gas outlet 3 and recovered from a rotary valve 8.
The spray drying apparatus of FIG. 10 is same as that of FIG. 9 except that as the atomizer, a rotary spraying disc 10 is used in place of the nozzle 5.
In the spray drying apparatuses of the type as shown in FIG. 9 and FIG. 10, a fine powder as a product is recovered in two portions from the apparatus main body (drying chamber) and the cyclone, respectively. In this case, the portion of the product having a heavier weight and larger particle diameters are taken out from the apparatus main body (drying chamber) and the portion of the product having a lighter weight and smaller particle diameters from the cyclone. Hence, there occurs a slight nonuniformity in product composition.
This nonuniformity in product composition has hitherto presented no serious problem, depending upon the kind of the product.
Meanwhile, recently in the spray drying processes and the apparatuses therefor, use of a more grade and smaller quantity production system in addition to the conventional less grade larger quantity production system has become more frequent than before for the production of medicines, fine ceramics, etc.
Moreover, the production of higher purity products has become more frequent in said processes and apparatuses.
Under the increased requirements for the more grade smaller quantity production system as well as for higher purity products, a new problem has arisen in addition to the slight nonuniformity in product composition which has not been considered as a major problem.
That is, the more grade smaller quantity production system requires more frequent product change, which in turn requires rapid and complete cleaning of a solid which has adhered to the inner wall of a drying chamber. Such cleaning is necessary because (1) efficient operation of the more grade smaller quantity production system requires a cleaning time as short as possible and (2) production of higher purity and higher quality products requires complete cleaning of an adhered solid at each product change. In actuality, complete cleaning is extremely difficult. Moreover, when the product is harmful to human bodies, direct contact with the product must be avoided making the cleaning very difficult.
Further, the slight nonuniformity in product composition has become a more serious problem in connection with the increased requirement for higher purity products.